I can do the weight and ballance of a 150 no problem.

The POH says that its ok to do it when the seat is so my inches forward and aft of the CG.

How about if you put your seat more forward or aft then the POH specifies?

The range for the seat arm (33 to 41 in) is determined by the length of the seat track so it is impossible to have the seat outside this range.

Ok.

But what if my seat was full back and my passengers is full forward?

The POH says its based on average occupant.

Thanks,

If you are being really picky you can determine the seat position for each occupant and use the exact inches.
Then you must consider fat legs against thin ones and the distribution of the body mass.

I think that providing your average calculations lie within the weight and balance limits and are not close to or on either boundary then there's room for the small deviations from exactitude your calculations will be.

In a Cessna 150, providing the people and baggage are within the weight limits you can't be too far out.
Two fat people means less baggage or no baggage and so there should not be a W+B problem
Two little people are worse because they can now carry a lot of baggage, and I think the baggage position will become more important to you.

In many aeroplanes you need to take ballast if the pilot is too light.
In the Cessna 206 with Wipline amphibs and the Seneca series putting ballast in the rear is essential! Don't assume that the aeroplane is within W+B with two people on board.
I was shocked to find the amphib 206 was well forward of the forward limit with two on board, only after I had landed it flat with power assist I put a 20 litre plastic bottle full of water in the very back after this incident!

I wish the ballast in the Seneca could have come in the form of bigger ailerons

Never seen a Seneca with spades on the ailerons -
might be an idea, though!

The Seneca is always wonky in the flare and hold off, it never seems to be happy.
I don't think spades will work very well on it... Probably better to change the wings entirely to a better section than the old Cherrytree one.